In recent years, a high power internal combustion engine has been developed with improved charging efficiency obtained by means of an intake system which takes advantage of kinetic effects, such as resonator effects and inertial effects, of intake air. Such an intake system is typically provided with a collector chamber, such as a surge tank, into which intake air introduced by an air cleaner is delivered through an intake passage and from which the intake air is distributed to the cylinders through separate or discrete intake passages.
In the intake system, the collector chamber serves as a primary resonator to generate pressure vibrations due to negative pressure produced by each cylinder in its intake cycle in a speed range wherein engine speeds are lower than a certain critical speed so as to obtain high charging efficiency by virtue of resonance effects. The collector chamber also serves as a space open to the atmosphere to invert negative pressure waves, generated in a downstream end portion of each discrete intake passage upon the opening of intake valve, to positive pressure waves in a speed range wherein engine speeds are higher than the certain critical speed so as to obtain high charging efficiency by virtue of inertial effects.
Typically used as such collector chambers are elongated cylindrical surge tanks and such an elongated surge tank is communicated with an upstream or main intake passage connected to either one of end walls and side wall thereof and with discrete intake passages arranged side by side and connected to the side wall. For this reason, the intake system of this type unavoidably has unequal-length discrete intake passages. This leads to unequally distribution and supply of intake air among the cylinders or to unequal resonance effects or inertial effects among the cylinders. The intake system, because of a rapidly changing path of intake air flow between the upstream intake passage and each discrete intake passage, increases the resistance of intake flow.
In an attempt to overcome such drawbacks as is described above, some intake systems have discrete intake passages whose ends clustered or grouped together at their upstream ends approximately symmetrically around a center axis of the cylindrical collector chamber and connected to the collector chamber. Such an intake system is known from Japanese Unexamined Utility Model Publication No. 60-88062.
In the intake system having a relatively small volume of collector chamber and discrete intake passages clustered or grouped together, since damping in intake air vibration propagated through the discrete intake passage is hard to occur, the discrete intake passages, as well as the collector chamber, can serve as an effective resonator space.
However, the intake system still has such shortcomings that the total volume of the discrete intake passages and collector chamber is too small to provide sufficient resonance supercharging effect, that because resonance pressure waves in the intake passage interfere with intake air before propagated into the collector chamber and therefore significantly damp, a reduction in resonance supercharging effect is unavoidable, and that because, although inertial supercharging effect for each cylinder is enhanced by the collector chamber and the discrete intake passages for the other cylinders serving as a space or chamber open to the atmosphere, the small volume of collector chamber adversely effects to greatly improving inertial supercharging effects.